src/Scenery/tilemgr.cxx: use simgear::Torrent's new terrasync utility fns.
simgear/io/torrent.*: add specific terrasync utility fns.
src/Translations/TranslationUnit.cxx: add missing include of <cassert>.
src/Scenery/tilemgr.cxx: optionally use Torrent subsystem for terrain
src/Main/fg_init.cxx: add simgear's new "torrent" subsystem.
simgear/io/torrent.*: in license, change GPL to LGPL.
simgear/io/torrent.*: new torrent sub-system.
simgear/io/HTTP*: give access to success/failure of final HTTP result.